Fuel cells produce electricity by converting reactants such as hydrogen and oxygen into products such as water. A fuel cell comprises a negative electrode, called a cathode; a positive electrode, called an anode; and an electrolyte situated between the two electrodes. During operation a voltage is produced between the anode and the cathode.
One fuel cell system uses an anode containing platinum, polymer electrolytes, and fuels derived from liquid hydrocarbons. The liquid hydrocarbon fueled fuel cell scheme is a promising power source for electric vehicles because its fuels are readily available, inexpensive, and easily transported. A partial oxidation reaction chemically transforms the hydrocarbons into the desired reactant, hydrogen, and into undesirable carbon monoxide and nitrogen byproducts. The hydrogen ions present at the anode travel across a polymer electrolyte to the cathode. Upon reaching the cathode, the hydrogen ions react with oxygen present at the cathode and electrons from the external circuit to produce water and an external electric current produced by the voltage difference between the anode and cathode.
Increasing the voltage between the anode and cathode is one way of enhancing a fuel cell's performance. Such a voltage increase can be obtained when the fuel cell electrodes are formed from catalytic materials. However, when catalytic poisons such as CO are present in the fuel, the anode to cathode voltage decreases. This in turn undesirably reduces the current flowing in the external circuit.
Hydrogen-oxygen fuel cell having platinum-containing catalytic anodes exhibit a measurable decrease in fuel cell voltage in cases where CO levels exceed about 1 to 5 ppm in the hydrogen fuel. It is believed that this decrease (referred to as an activation overpotential) is caused by the additional electric potential needed at the anode to oxidize the carbon monoxide into carbon dioxide. As electric current is made available to the external circuit, the overpotential increases, and consequently decreases the fuel cell's effectiveness as a generator of electric energy.
Some methods for reducing the effect of CO poisoning of fuel cell electrodes such as the water gas shift reaction and preferential partial oxidation concentrate on processing the hydrogen fuel so as to remove as much CO as possible. However, even when preferential oxidation and water gas shift are used in combination under transient conditions, those processes result in a hydrogen fuel containing excessive CO impurities.
Other methods for reducing the effect of CO impurities on fuel cell voltage use CO-tolerant fuel cell electrodes. The amount of activation overpotential that develops at an electrode in the presence of CO impurities depends on the electrode potential that the anode requires to oxidize the adsorbed carbon monoxide. Changing the composition, electronic structure, and physical structure of the anode material can affect the potential required to oxidize the carbon monoxide.
Both Pt/Ru and Pt/Sn electrodes are known to exhibit CO oxidation activity at potentials lower than those observed with pure platinum electrodes. However, it is believed that electrodes made from these materials cannot tolerate CO concentrations in the hydrogen fuel in excess of about 10 ppm without exhibiting CO activation polarization. This CO tolerance is less than that needed for practical fuel cell use. Further, the observed CO activation polarization results in a 200 to 500 mV reduction in fuel cell voltage in a cell made with electrodes fabricated using these materials, thereby reducing the cell's effectiveness as an electric power generator.
Platinum particles dispersed in non-stoichiometric hydrogen tungsten bronzes also have been used as electrodes for fuel cells (see for example, U.S. Pat. No. 5,470,673).
Consequently there is a continuing need for other CO-tolerant anodes that are capable of oxidizing carbon monoxide at low potentials.